Removing material from a substrate to form microscopic or nanoscopic structures is referred to as micromachining. Removing material is also referred to as milling or etching. Laser beams and charged particle beams are used for micromachining. Each has advantages and limitations in various applications.
Laser systems use several different mechanisms for micromachining. In some processes, the laser is used to supply heat to a substrate to induce a chemical reaction. The reaction occurs only in the areas where the laser supplies heat, although the heat tends to diffuse to an area larger than the laser beam spot, limiting the resolution of the process. Another mechanism used in laser micromachining is photochemical etching, in which the laser energy is absorbed by individual atoms of the substrate, exciting them into a state in which they can chemically react. Photochemical etching is limited to materials that are photochemically active. Another mechanism used in laser machining is laser ablation, in which energy supplied rapidly to a small volume causes atoms to be expelled from the substrate without heating the substrate. Laser ablation using a fast-pulsed femtosecond laser is described, for example, in U.S. Re. 37,585 to Mourou for “Method for controlling configuration of laser induced breakdown and ablation.” Femtosecond laser ablation overcomes some of the limitations of the processes described above.
Charged particle beams include ion beams and electron beams. Ions in a focused beam typically have sufficient momentum to micromachine by physically ejecting material from a surface. Because electrons are much lighter than ions, electron beams are typically limited to removing material by inducing a chemical reaction with an etchant. Ions beams typically are generated by a liquid metal ion source or by a plasma ion source. The spot size of a charged particle beam depends on many factors, including the type of particles and the current in the beam. A beam with low current can typically be focused to a smaller spot and therefore produce a smaller structure than a beam with high current, but a low current beam takes longer to micromachine a structure than a high current beam.
Lasers are typically capable of supplying energy to a substrate at a much higher rate than charged particle beams, and so lasers typically have much higher material removal rates than charged particle beams. The wavelength of lasers, however, is much larger than the wavelength of the charged particles in the charged particle beams. Because the size to which a beam can be focused is limited by the wavelength, the minimum spot size of a laser beam is typically larger than the minimum spot size of a charged particle beam. A. P. Joglekar et al., in “Optics at Critical Intensity: Applications to Nanomorphing,” Proceedings of the National Academy of Science, vol. 101, no. 16, pp. 5856-5861 (2004) (“Joglekar et al.”) shows that features smaller that the wavelength can be achieved using laser pulses shorter than about 10 picoseconds near the critical intensity for ionization. The feature size achievable by Joglekar et al. is still not sufficiently small for many nanotechnology applications.
While a charged particle beam typically has greater resolution than a laser beam and can micromachine an extremely small structure, the beam current is limited and the micromachining operation can be unacceptably slow. Laser micromachining, on the other hand, can be faster, but the resolution is inherently limited by the longer wavelength.
One way to take advantage of both the faster micromachining capability of lasers and the higher precision of charged particle beams is to sequentially process a sample. Sequential processing is described, for example, by M. Paniccia et al. in “Novel Optical Probing and Micromachining Techniques for Silicon Debug of Flip Chip Packaged Microprocessors,” Microelectronic Engineering 46 (pp. 27-34 1999) (“Paniccia et al.”). Paniccia et al. describe a known technique for accessing the active portion of a semiconductor flip chip using laser-induced chemical etching to remove the bulk of material, and then using a charged particle beam for the final, more precise micromachining. A problem with sequential processing is determining when to stop the faster, less precise laser micromachining and begin the more precise charged particle beam processing. If the laser processing is stopped too soon, excess material will remain for removal by the charged particle beam. If the laser processing is stopped too late, the work piece will be damaged. Determining when to stop processing is referred to as “endpointing.”
Techniques for determining the end point in charged particle beam processing are known and described, for example, in U.S. Pat. Pub. 2005/0173631 to Ray et al. Such techniques include, for example, applying a varying voltage to the underlying circuit to change the secondary particle emission when the underlying circuit is exposed or nearly exposed. By observing the secondary particle emission, an operator can determine when a feature, such as a buried conductor, is uncovered. Other charged particle beam endpointing processes include, for example, detecting transistor leakage current caused by the charged particles injected by the beam. Laser processing is typically not performed in a vacuum chamber, and so secondary electrons and ions cannot be collected.
In ion beam processing, it is also known to detect photons of a specified frequency emitted from the substrate to determine when the material being impacted by the ion beam has changed. Such a process is described, for example, U.S. Pat. No. 4,874,947 to Ward et al. for “Focused Ion Beam Imaging and Process Control,” which is assigned to the assignee of the present application. While Ward et al. describe the detection of photons for endpointing in an ion beam system, this technique is not widely used because the low photon signal is difficult to collect.